1. Field of the Invention
This invention is related to the field of concentration enhancement of a volatile product. More specifically, this invention is directed to a method and an apparatus to increase the concentration of a volatile product that is present at too low a concentration for conventional purification methods, such as distillation, to be economical.
2. Description of the Related Art
There is a current need to find alternate energy sources to substitute for the use of fossil fuels for transportation applications. For example, biologically produced ethanol has been proposed as an alternative to petroleum-derived liquid fuels. See U.S. Pat. No. 6,306,639: ethanol derived from living organisms is an attractive alternative to petroleum based fuels because it is a renewable resource. Other biologically-produced molecules, such as butanols, have also been proposed as fuels. Furthermore, such biologically-produced molecules may have applications as chemical feedstocks. A common issue in the production of these biological molecules is the separation, and purification, of the target biological molecule from other species present in the synthesis of the target. The invention disclosed herein is directed to the objective of separation and purification of target molecules.
The work described herein gives examples with ethanol, but the method could be used for other volatile target molecules, whether produced biologically or otherwise.
The problem of separating target molecules present in dilute concentrations has been recognized in the art. Vane and Alvarez in published U.S. Application 20090057128 (11/896,201) disclose a method for separation of liquid mixtures involving vapor stripping followed by mechanical compression of the vapor which is then exposed to a permeation membrane for separation of the compressed vapor. The method of Vane and Alvarez, unlike that of the instant invention, requires the use of a membrane. Furthermore, the compressor in Vane/Alvarez serves a different purpose. See also Vane, Alvarez, Huang, and Baker, “Experimental validation of hybrid distillation-vapor permeation process for energy efficient ethanol-water separation,” Journal of Chemical Technology and Biotechnology, 2009. [“The energy demand of distillation-based systems for ethanol recovery and dehydration can be significant, particularly for dilute solutions. An alternative separation process integrating vapor stripping with a vapor compression step and a vapor permeation membrane separation step, termed membrane assisted vapor stripping (MAVS), has been proposed.”] In MAVS, the purpose of the compressor is to force ethanol-water vapor through a selective membrane to reutilize the water vapor in the steam stripper. The compression pressure required is related to the membrane area (and its permeability), and in order to reduce the capital cost of the membrane, may be relatively high. In the present invention, the generation of steam for the stripper is almost completely through the recovery of heat from the condensed product, so the purpose of the compressor is just to raise the saturation temperatures a few degrees higher than the evaporator. See also M. P. Taylor, et al., “Thermophilic ethanologenesis: future prospects for bioethanol production,” Trends in Biotechnology, 2009, 27, 398405. Separately, note PCT/US2010/043574, which describes separation using a dephlegmator (a partial condensing heat exchanger); such a process lacks the heat integration benefits of the present invention, which are more fully described below.
Other background material may be found in U.S. Pat. Nos. 4,769,113, 5,772,850, 5,968,321, and 6,899,743 (mentioning the use of a dephlegmator) and published US applications 20070031954 and 20090215139 (ethanol recovery process). On vapor compression in desalination applications, see M. T. Ravanchi, et al., “Application of membrane separation processes in petrochemical industry: a review,” Desalination, 2009, 235, 199-244. Also note: X. Lu, “A perspective: photosynthetic production of fatty acid-based biofuels in genetically engineered cyanobacteria,” Biotechnology Advances, 2010, 28, 742-746; D. C. Ducat, et al., “Engineering cyanobacteria to generate high-value products,”—Trends in Biotechnology, February 2011, Vol. 29, No. 2, pp. 95-103; M. S. Elshahed, “Microbiological aspects of biofuel production: current status and future directions,” Journal of Advanced Research (Cairo), 2010, 1, 103-111; J. A. Viera Costa and M. Greque de Morais, “The role of biochemical engineering in the production of biofuels from microalgae” Bioresource Technology, 2011, 102, 2-9; S.-H. Ho, et al., “Perspectives on microalgal CO2-emission mitigation systems—a review,” Biotechnology Advances, 2011, 29, 189-198.
The present invention in vapor compression steam stripping [VCSS] is related to the following observations.
(1) The Determinant of Purification Cost from Low Concentration Feeds is the Cost of the First Stage of Purification.
Consideration of the observed cost of distillation as a function of input ethanol concentration reveals that cost increases rapidly at low concentrations. Distillation occurs in stages, and at low concentration the gain in ethanol/water ratio is quite large and can exceed 10. Thus, for example, the total cost of distillation from 0.5% is the sum of the cost of a single stage of distillation that yields about 5% ethanol, and the cost of distillation of 5% ethanol to fuel grade. Since the cost of distillation from 5% to fuel grade is acceptable, if the cost of the first stage can be reduced to a small value similar to the cost per stage of later stages, then total cost will be acceptable. VCSS, unlike other attempts to reduce the cost of ethanol purification from dilute solutions, solves the correct problem: cost of the first stage.
(2) Vapor Compression can be Used to Recycle the Heat of Vaporization of Water.
The reason distillation is so expensive is that it is necessary to evaporate so much water, which takes a lot of energy. Vapor compression is widely used for heat recycling in water distillation. In the desalination industry, the vapor compression of the product (steam) is routinely utilized, with an average energy consumption of 30 wh per gallon of distilled water (Miller JE (2003): Review of water resources and desalination technologies, Sandia National Laboratories, SAND 2003-0880, Albuquerque, N. Mex.}. The lowest energy consumption is reported by AB Welding's desalination (water distillation) units at 15 wh/gal for units 1000 GPH and larger. (GPH denotes gallons per hour. This data is available from AB Welding, 2597 North Fordham Avenue, Fresno, Calif. 93727-8601 on request). By coupling the most energy efficient of these units with steam stripping, it is possible to concentrate ethanol from very dilute feeds to levels that would make standard distillation economically viable, while using a fraction of the energy of the latter. Conceptually, the energy taken by the compressor can be recovered through heat exchangers within the VCSS.
(3) Combining Steam Stripping and Vapor Compression
By combining steam stripping and vapor compression, it is possible to design an ethanol purification process that is continuous, removes the ethanol from an input liquid stream as completely as one wants, with a vapor phase as close to equilibrium with the input liquid stream as one wants, with an energy requirement that approaches the theoretical minimum.
In one embodiment, the present invention can be viewed as an adjunct to distillation or other methods of purification, not necessarily as an alternative.